1. Field of the Invention
The present invention relates to a membrane electrode assembly having a pair of gas diffusion electrode layers on each side of a solid polymer electrolyte membrane, and a fuel cell having the membrane electrode assembly sandwiched by a pair of separators. In particular, the present invention relates to a membrane electrode assembly or a fuel cell having a solid polymer electrolyte membrane extending over one gas diffusion electrode layer.
2. Description of Related Art
Some types of fuel cell have a structure in which a fuel cell unit comprises a membrane electrode assembly which is sandwiched by a pair of separators, and a plurality of the fuel cell units are stacked.
An example of such a membrane electrode assembly is explained with reference to FIG. 9. In the drawing, a reference numeral 1 indicates a membrane electrode assembly. The membrane electrode assembly 1 comprises a solid polymer electrolyte membrane 2 and gas diffusion electrode layers 3 and 4 (an anode gas diffusion electrode layer 3, a cathode gas diffusion electrode layer 4) which are disposed on both sides of the solid polymer electrolyte membrane 2. In the gas diffusion electrode layers 3 and 4, catalyst layers 5 and 6, and gas diffusion layers 7 and 8 are formed. The catalyst layers 5 and 6 contact both sides of the membrane electrode assembly 2. As shown in FIG. 9, the planar dimension of the solid polymer electrolyte membrane 2 is larger than the planar dimension of the gas diffusion electrode layers 3 and 4 disposed on both sides of the solid polymer electrolyte membrane 2. A portion of the solid polymer electrolyte membrane 2 extends to an outer circumferential region of the gas diffusion electrode layers 3 and 4. On both sides of the membrane electrode assembly 1 having such a structure, a pair of separators (not shown in the drawing) are disposed. Ring-shaped sealing members (not shown in the drawing) are disposed near a peripheral portion of the separator facing each other; thus, a fuel cell unit is formed.
In the fuel cell unit having such a structure, when a fuel gas (for example, a hydrogen gas) is supplied to a reactant surface of the above-mentioned anode gas diffusion electrode layer 3, hydrogen is ionized in a catalyst layer 5 of the anode gas diffusion electrode layer 3 so as to be transmitted to a catalyst layer 6 of a cathode gas diffusion electrode layer 4 via a solid polymer electrolyte membrane 2. An electron which is generated during such a transmission is extracted to the outside of the membrane electrode assembly and is utilized as a direct current electric energy. An oxidizing gas (for example, air containing oxygen) is supplied to the cathode gas diffusion electrode layer 4; thus, a hydrogen ion, an electron, and oxygens react so as to generate water.
Examples of other type of membrane electrode assembly are shown in FIGS. 10 and 11. In a membrane electrode assembly 1 shown in FIG. 10, a solid polymer electrolyte membrane 2 and gas diffusion electrode layers 3 and 4 are formed in the same size having the same ends and layered (See U.S. Pat. No. 5,176,966). In a membrane electrode assembly 1 shown in FIG. 11, gaskets 10 and 11 are disposed between the solid polymer electrolyte membrane 2 and the gas diffusion electrode layers 3 and 4 so as to seal end portions of the solid polymer electrolyte membrane 2 by the gaskets 10 and 11 (See U.S. Pat. No. 5,464,700).
However, conventional membrane electrode assembly has the following problems.
Recently, it is demanded that the sizes of a fuel cells be reduced. In order to supply such a fuel cell, thickness of a solid polymer electrolyte membrane in a membrane electrode assembly tends to be thinner. When thickness of a solid polymer electrolyte membrane 2 in a membrane electrode assembly 1 shown in FIG. 1 is reduced, there is a concern that strength of a portion of the solid polymer electrolyte membrane 2 which extends over gas diffusion electrode layers 3 and 4 may decrease.
Furthermore, in a membrane electrode assembly 1 shown in FIG. 9, a solid polymer electrolyte membrane 2 receives stress from the outer circumferential end of catalyst layers 5 and 6 to the same regions on both sides of the solid polymer electrolyte membrane 2; thus, there is a concern that excessive stress occurs thereon.
Also, in a membrane electrode assembly 1 shown in FIG. 10, both ends of the gas diffusion electrode layers 3 and 4 which are disposed on both sides of the solid polymer electrolyte membrane 2 coincide both ends of the solid polymer electrolyte membrane 2. Thus, reactant gases which are supplied to the gas diffusion electrode layers 3 and 4 tend to diffuse to the outside thereof. Therefore, there is a concern that the reactant gases may be mixed near the ends of the gas diffusion electrode layers 3 and 4. Furthermore, there is a concern that the ends of the gas diffusion electrode layers 3 and 4 are so close that short circuiting will occur.
Also, in a membrane electrode assembly 1 shown in FIG. 11, gaskets 10 and 11 are disposed near an end region between the gas diffusion electrode layers 3 and 4 and the solid polymer electrolyte membrane 2; therefore, thickness of the end region increases. Also, the gas diffusion electrode layers 3 and 4 is bent and loses flatness due to contacting the gaskets 10 and 11. Thus, manufacturing process of the membrane electrode assembly 1 becomes complicated because a countermeasures must be taken to maintain flatness.
An object of the present invention is to provide a membrane electrode assembly and a fuel cell in which the thickness of the solid polymer electrolyte membrane is thin by enhancing self-protection of the solid polymer electrolyte membrane.